A current common requirement for an advanced electronic circuit and particularly for circuits manufactured as integrated circuits (“ICs”) in semiconductor processes is the use of a solder bump, solder ball or solder column connections. In a “flip chip” approach to packaging and interconnections, the solder bumps are used to couple the external terminals of a monolithic integrated circuit (which may be a silicon substrate with active or passive circuit elements and connections formed upon it, or other substrate materials including gallium arsenide (GaAs) and silicon on insulator (SOI), and silicon germanium (SiGe) may be used) to a package substrate or circuit board. These integrated circuit devices may have tens or hundreds of input and output terminals for receiving, and sending, signals and/or for coupling to power supply connections. In some IC designs the terminals are placed at the periphery of the integrated circuit and away from the active circuitry. In more advanced and complex integrated circuits, the terminals may be placed over the active area and lie over the active devices. In memory ICs, sometimes a center pad arrangement is used.
In a “flip chip” application, the integrated circuit (IC) is sometimes mounted face down (flipped). Terminal openings are formed in a protective insulating layer, called a passivation layer that overlies the face of the device. The input/output terminals are exposed and solder bumps, solder columns or solder balls are placed on these terminals, usually referred to as pads or lands, which may be formed as columns of conductive material extending away from the surface of the integrated circuit. The solder balls are then used to form the external connections to the integrated circuit. These solder balls may be provided already formed on the completed integrated circuit, using a “wafer scale” approach, or, may be added later after the devices are singulated into individual devices called “dice”, or even disposed on a substrate or circuit board, and the integrated circuit then aligned with these balls and carefully placed on it. In any case, a thermal solder reflow process is typically used to complete the solder connection between the flip chip integrated circuit, and a substrate, film, printed circuit board or even another silicon device. In thermal reflow the solder bumps, balls, or columns form a permanent mechanical and electrically conductive connection between the terminals of the integrated circuit and the substrate. The combined flip chip IC and substrate may then be packaged as a single integrated circuit. Alternatively, in a multiple chip module form, the flip chip may be combined with other integrated circuits which may also be “flip chips” or wire bond connections may be used. For example sometimes memory devices such as FLASH nonvolatile devices, and processors that would use the FLASH device for program or data storage, are combined in a single packaged device. IC devices may be stacked vertically, or placed alongside one another using a larger substrate or circuit board.
Recently the semiconductor industry has been moving to “lead (Pb) free” packaging and device connector technology. This trend increasingly results in the use of lead free solder bumps and lead free solder balls to form connections with integrated circuits and packages. These lead free solder materials are formed of tin and tin alloys which may include, for example, silver, nickel, copper and other metals. The use of lead free solder balls is safer for the environment, safer for workers in the industry and safer for consumers than lead based solder bumps or solder balls. However, the quality and reliability of the final solder connections formed has not always been as great as desired. During thermal cycle (TC) testing, for example, cracks in the completed solder connections have been observed. A typical thermal cycle test will perform 500 thermal cycles (TC 500) ranging from the minimum specified temperature (typically −55 degrees Celsius) to the maximum specified temperature (typically +125 degrees Celsius), and then the solder connections may be inspected. Often following TC500 testing using conventional solder lead free solder connections, ball cracks are found. Ball cracks are an indicator that during actual use in an application, a solder connection is going to fail by pulling apart. Clearly this is unacceptable in a useful product.
Further, recent advances in interconnection technology for integrated circuit have caused a transition from aluminum or aluminum alloy conductors used within the integrated circuit and extending to the terminals, to copper based interconnect technologies. Often copper conductors are formed over the substrate using a damascene or dual damascene approach. In this technology, a trench or trench and via combination is formed in an insulation dielectric material that forms inter-layer dielectric (ILD) or inter-metal dielectric (IMD), a seed layer of copper or copper alloy is formed within the trench, electroplating or electroless chemical deposition (ECD) is used to fill the trench and/or via opening with the copper based interconnect material, and sometimes a chemical mechanical process (CMP) tool is used to mechanically and chemically abrade any excess copper material above the surface of the trench to form a planar surface at the top of the trench or via. In this way the pattern of the trench in the dielectric finally determines the pattern of the conductors. Many levels of interconnect may be formed above the transistors and active devices, which are typically formed at the surface of the underlying semiconductor substrate. The interconnect material of copper or copper alloy may extend to the terminal pads or ball lands or columns of the integrated circuit, and in such a conventional case copper or an alloy of copper is the material that the solder bumps or balls would be then be deposited upon.
TC500 tests and analysis of lead free solder balls or bumps formed on copper pads indicate that the use of copper as a metallization material at the solder ball terminals of the integrated circuit and/or on the trace of the corresponding circuit board that receives the flip chip integrated circuit may cause the balls to exhibit ball cracks. During solder reflow, which is a thermal process, an intermetallic compound (IMC) is formed within the solder ball between the integrated circuit terminal, which is a copper pad or column, and the solder materials. However if the ball does not adhere well at the interface between the solder and the IMC, then ball cracks are likely to occur.
Accordingly, methods and apparatus for packaging integrated circuits and for forming low resistance electrical connections to circuit boards that form robust and high quality electrical and mechanical connections, while using lead free solder materials, are needed.